The present invention relates to a novel polypropylene film usable as a substitute for soft vinyl chloride films. More particularly, the present invention relates to a novel polypropylene-based film that is less tacky, well-balanced between a tensile modulus and a heat seal temperature, and excellent in processability, transparency and impact resistance. The present invention further relates to a novel multilayered polyolefin resin laminate, and more particularly, to a novel multilayered polyolefin resin laminate that is well-balanced between a tensile modulus and a heat seal temperature, less tacky, and excellent in slipping and anti-blocking properties necessary for high-speed bag forming.
Vinyl chloride resins are widely used as soft resin. However, it is known that the vinyl chloride resins generate harmful substances upon burning. For this reason, it has been strongly required to develop a substitute resin. Recently, there have been used, as a substitute resin, olefin polymers produced by using a metallocene catalyst. Example of these olefin polymers include a copolymer of ethylene and xcex1-olefin. However, such a copolymer poses a problem that it becomes too tacky when softened. Further, shaped articles, such as film, produced from the copolymer have poor surface properties because of deteriorated transparency, rigidity and low-temperature heat sealability, thereby rendering it unsatisfactory. The copolymer is also poor in processability.
Crystalline propylene polymer films are widely used as a packaging film owing to their excellent rigidity, transparency and moisture resistance. The crystalline propylene polymer films may be used alone as monolayered films, or in combination with films of ethylene-vinyl acetate copolymer, linear polyethylene or the like as multilayered laminate films. For their use, the packaging films are often processed into bags, regardless of whether the films are monolayered or multilayered. A series of processes for forming a film into a bag and closing the opening of the bag after putting contents therein is generally carried out by pressing the film edges with a heated rod and melt-welding the edges to each other (so called xe2x80x9cheat sealingxe2x80x9d). Recently, it has been required to speed up the bag-forming and packaging processes for increased productivity, which results in great demands for materials with good heat sealability. Also, it has been required for the film to inevitably have slipping and anti-blocking properties so that the above fabrication process can be efficiently made. It has been known that a multilayered laminate fails to show a sufficient interlaminar strength when produced without using an adhesive and the like. It is therefore necessary to adhere layers of a laminate by using the adhesive in order to impart excellent properties of low-temperature heat sealability, rigidity, transparency, moisture resistance and the like to the multilayered laminates.
Thus, a first object of the present invention is to provide a polypropylene film that is less tacky, well-balanced between a tensile modulus and a heat seal temperature, and excellent in processability, transparency and impact resistance. A second object of the present invention is to provide a multilayered polyolefin resin laminate that is well-balanced between a tensile modulus and a heat seal temperature, less tacky, and excellent in slipping and anti-blocking properties necessary for high-speed bag forming.
As a result of extensive research, the inventors have found that the above objects are achieved by a polypropylene film satisfying a specific relationship between a tensile modulus TM (MPa) and a heat seal temperature HST (xc2x0 C.). The inventors have further found that the above object is achieved by a multilayered polyolefin resin laminate in which at least one outermost layer is made of the above polypropylene film. The present invention has been accomplished based on these findings.
Thus, according to a first aspect of the present invention, there is provided a polypropylene film satisfying the following formula:
TMxe2x89xa712.5xc3x97HSTxe2x88x92900
wherein TM is a tensile modulus (MPa) and HST is a heat seal temperature (xc2x0 C.).
The above polypropylene film may be produced from a propylene polymer (A) having:
(1) a melting point Tm of from 120 to 135xc2x0 C. as measured by differential scanning calorimeter (DSC);
(2) a melting endotherm xcex94H (J/g) as measured by the DSC, satisfying the following formula:
xcex94Hxe2x89xa70.45xc3x97Tm+22;
(3) a half-width Th (xc2x0 C.) of the peak top of an elution curve as measured by temperature rising fractionation, of 5xc2x0 C. or lower; and
(4) an intrinsic viscosity [xcex7] of from 1 to 3 dl/g as measured at 135xc2x0 C. in tetralin.
As the propylene polymer, preferred is a propylene homopolymer (A-1) satisfying:
(1) a meso pentad fraction [mmmm] of from 70 to 80 mol %; and
(2) a racemic pentad fraction [rrrr] satisfying, together with 1-[mmmm], the following formula:
[rrrr]/(1xe2x88x92[mmmm])xe2x89xa60.1.
The propylene polymer (A) and the propylene homopolymer (A-1) are produced by polymerizing propylene, or polymerizing propylene with ethylene and/or C4-C20 xcex1-olefin, in the presence of a specific transition metal catalyst.
According to a second aspect of the present invention, there is provided a multilayered polyolefin resin laminate, wherein at least one outermost layer is made of the above polypropylene film. Other laminate layers are preferably produced from a crystalline propylene polymer other than the propylene polymer (A) and the propylene homopolymer (A-1).